As shown in FIG. 12A and FIG. 12B, an optical transceiver 100 used for the optical communication includes an optical module 101, a printed circuit board 102, an electric connector 103, a case 104, and others. FIG. 12A is a top view of the optical transceiver 100, and FIG. 12B is crosses sectional view along arrow direction at line H-H of the optical transceiver 100 in FIG. 12A.
The optical module 101 includes a laser element which outputs an optical signal, and an optical receiving element which receives the optical signal. In addition, the optical module 101 includes a signal terminal and a power supply terminal which are connected to the laser element and the optical receiving element.
Hereinafter, the signal terminal and the power supply terminal are named the module terminal generically. The optical module 101 is mounted nearby edge of the printed circuit board 102, and the electric connector 103 is formed nearby the other edge. The module terminals 114 of the optical module 101 are inserted into through-hole terminals 115 that are formed in the printed circuit board 102 and are soldered after that. As a result, the optical module 101 is electrically connected with pattern wirings of the printed circuit board 102. Further, the peripheral circuit 105 such as a driving circuit for driving the optical module 101 is mounted on the printed circuit board 102.
The case 104 includes a board mount part 110 for receiving the printed circuit board 102 which mounted the optical module 101, an optical cable mount part 111 with which an optical cable (not shown) is connected, and a communication device mount part 112 with which the communication device (not shown) is connected. Further, the printed circuit board 102 is mounted in the board mount part 110 so as an optical axis Q of the optical module 101 and the optical axis of the optical cable may coincide when the optical cable is attached to the optical cable mount part 111. When the printed circuit board 102 is mounted in the board mount part 110, the electric connector 103 is projected from the communication device mount part 112. A distance between a top surface 102a of the printed circuit board 102 and a top surface K1 of the communication device mount part 112 is predetermined height H1. This height H1 is defined in a specification called MSA (Multi-Source Agreement).
Because the electric connector 103 and the optical module 101 are mounted on the printed circuit board 102, a difference of height H2 between a top surface 102a of the printed circuit board 102 and a top surface K2 of the optical cable mount part 111 is set according to height of the optical module 101. When height of the optical module 101 becomes high, that is, when height H2 becomes high, height H3 of the case 104 becomes high.
Japanese Patent Application Laid-Open No. 2007-67380 discloses an optical transceiver where a printed circuit board and an optical module are mounted on a flexible board.
By using this kind of the flexible board, even if the optical module 101 and the electric connector 103 are set in a different height, it can absorb difference of height between the optical module 101 and the electric connector 103 since the flexible board is deformed. Accordingly, it is possible to lower height of the case 104.
On the other hand, when the module terminals 114 of the optical module 101 are soldered in the through-hole terminals 115 of the printed circuit board 102, a variation may occur to insertion amount of the module terminals 114 that are inserted in the through-hole terminals 115. A variation of the insertion amount of the module terminals 114 is equivalent to a variation of height H4 of the optical module 101 that is measured from the top surface 102a of the printed circuit board 102. Accordingly, the height H3 of the case 104 has to be set considering the variation amount of the height H4 of the optical module 101. Therefore, the height H3 of the case 104 is increased by the variation amount of the height H4 of the optical module 101.
Japanese Patent Application Laid-Open No. 2007-108542 discloses an optical fiber module wherein an optical module is surface mounted on a printed circuit board.
When a surface mounting technology is applied, because height of the optical module 101 from surface of the printed circuit board 102 is almost constant, height of the case 104 of the optical module 101 can be defined without considering variation amounts of the height H4 of the optical module 101. Therefore, it is possible to minimize the height H3 of the case 104.
By the way, due to high-frequency signal that flows between the driving circuit and the optical module, return current flows among them. Therefore, transfer characteristics of the high-frequency signal degrade by the return current. However, Japanese Patent Application Laid-Open No. 2007-67380 and Japanese Patent Application Laid-Open No. 2007-108542 mentioned above does not refer the alignment such as distance between the optical module and the driving circuit. In the case when distance between the driving circuit and the optical module becomes long, because a current path of the high-frequency signal and the return current also becomes long in connection with the distance, degradation amount of the above-mentioned transfer characteristics becomes large.